The generic term “NOx” describes oxides of nitrogen, such as nitrogen monoxide (“NO”) and nitrogen dioxide (“NO2”). NOx is produced as a by-product of combustion in some engines. Aircraft engines, automobile engines, and power generators, for example, utilize combustion processes that produce NOx.
NOx is believed to contribute to the production of acid rain, smog, and the depletion of the ozone layer. For example, reactions including volatile organic compounds and NOx, occurring in sunlight, are believed to form ozone. Ground-level ozone is believed to contribute to throat irritation, congestion, chest pains, nausea and labored breathing. Such ozone is believed to aggravate respiratory conditions like chronic lung and heart diseases, allergies and asthma. Ozone is believed to effectively “age” lungs and may contribute to lung disease. With an increase in the number of vehicles, the amount of NOx produced is increasing, and is believed to exacerbate environmental harm.
For at least these reasons, it is desirable to monitor NOx gas concentrations in emissions. In fact, NOx sensing, such as for emission control, is a requirement of regulations governing the operation of some diesel and spark ignition engines. Regions applying such regulations include countries in North America and Europe. As an example, in 2010, the total production of emission-regulated heavy-duty trucks, which will be required to monitor NOx, will be at least 1.5 million. Similar emission standards are being proposed for other internal combustion engines (i.e., passenger cars, boats, sport vehicles, etc.). Accordingly, there is a need for a reliable NOx sensor to monitor and control emissions.